Confocal Laser Scanning Microscopy (CLSM) has been used as a fast, user-friendly, and noninvasive tool for characterizing the phase composition differences at the substrate and air interfaces in thick films of polymer blends. A clearly different phase composition at the blend/glass interface and at the blend/air interface has been detected. We show that PCBM preferentially accumulates at the glass/blend interface, while P3HT preferentially accumulates at the blend/air interface, by comparing the integrated signal intensity of the luminescence coming from both interfaces. Our results demonstrate that CLSM can be used conveniently for the fast identification of a preferential phase segregation at interfaces in polymer blends. This is useful in the research field on devices (like sensors or planar waveguides) that are based on very thick layers (thickness higher than 1？μm). 1. Introduction In the last few years, it has been widely demonstrated, using several complex experimental techniques, that many systems composed of organic blends undergo a vertical phase stratification. For instance, polymers blended with fullerenes like poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (APFO-3) [1, 2] where the dynamic secondary ion mass spectroscopy (SIMS) technique has been used for the characterization, or poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT) [3] using XRD and small-angle Neutron Scattering, or Poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) [4] as well as all-polymer blends like poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) and poly(9,9-di-n-octylfluorene-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene)) (TFB) [5]. The vertical stratification of the prototype poly(3-hexylthiophene) (P3HT) blended with -phenyl-C61-butyric acid methyl ester (PCBM) has been characterized by several experimental techniques. Tsoi et al. studied thin films of RR-P3HT？:？PCBM and RRa-P3HT？:？PCBM by UV and X-ray photoelectron spectroscopy (UPS and XPS) [6], demonstrating that the P3HT chains are preferentially ordered at the blend-air interface, while PCBM tends to accumulate at the substrate-blend interface. Xue et al. [7] investigated the effect of the annealing process on the vertical stratification of P3HT？:？PCBM blend by means of X-ray diffraction (XRD) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Karagiannidis et al. [8] also studied the
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